The use of steroids and drugs released in vivo to counter trauma caused by an implanted device such as a cardiac pacemaker lead is well known. Such trauma typically occurs in the region of attachment of the distal end of the pacing lead to the cardiac tissue.
Tined pacing leads which have a cavity or collar at the distal end of the lead containing a drug to counter undesirable interactions between the lead and tissue are disclosed in U.S. Pat. Nos. 4,711,251 and 4,506,680, to Stokes, U.S. Pat. No. 4,844,099, to Skalsky et al., and U.S. Pat. No. 4,972,848, to Di Domenico et al. The drug of Di Domenico et al. is compounded within a polymer matrix specifically chosen to be dimensionally stable, not expanding when hydrated. They suggest that the degree of crosslinking of the polymer may be useful in varying the elution rate.
Tined leads having tips consisting of porous or molecular sieve-forming material, with a drug being stored in and dispensed from the tip, are disclosed in U.S. Pat. No. 4,819,662, to Heil, Jr. et al.; in U.S. Pat. No. 4,606,118, to Cannon et al.; and in U.S. Pat. No. 4,577,642, to Stokes.
A screw-in pacemaker lead is disclosed in U.S. Pat. No. 4,819,661, to Heil, Jr. et al., which has a chamber open to the distal end of the lead. A matrix impregnated with a therapeutic drug is retained in the chamber. A screw-in lead with a drug impregnated matrix is disclosed in U.S. Pat. No. 4,953,564, to Berthelson, in which the drug elutes out by means of a porous sintered elution path. Because the matrix swells in use, an expansion space is provided.
An implantable, porous stimulating electrode with a thin coating of hydrophilic polymer in which is embedded an anti-inflammatory steroid is disclosed in U.S. Pat. No. 5,103,837, to Weidlich. In this system, the steroid simply diffuses from the polymeric layer into the adjoining tissue to reduce growth of connective tissue.
U.S. Pat. No. 4,711,251, to Stokes, mentioned above, includes an embodiment that uses an osmotic pump to control dispensing of the drug. It has two chambers separated from each other by an impermeable membrane. The inner chamber contains the drug and is adjacent the electrode; the outer chamber is separated from body fluids by a semipermeable membrane. According to the specification, body fluids will enter the outer chamber through the semipermeable membrane to impart a pressure on the inner chamber via the impermeable membrane, resulting in dispensing of the drug stored within the inner chamber through the electrode. However, it is unclear why body fluids would enter the outer chamber to the extent that pressure would be imparted to the inner chamber. Stokes does not describe anything within the outer chamber that would draw in fluid. A figure shows a fluid in the outer chamber; presumably, this is body fluid which has entered the outer chamber after implantation, since no other outer chamber fluid is mentioned.